The present invention relates to scanning electron microscopes (SEM); and more particularly to a scanning electron microscope which is provided with a color image display.
Scanning electron microscopes are known to be useful instruments in the observation and analysis of matter. The use of the SEM encompasses both scientific analysis and routine, industrialized quality monitoring of samples.
Generally, an SEM includes a vacuum chamber, an electron optical system for generating and focusing an electron beam (sometimes referred to as the "primary electron beam"), a deflection system for moving the beam across a sample in a predetermined pattern, a detector system for detecting phenomena from the sample caused by the impinging electron beam, and a display system. When the electron beam strikes the sample, a complex response is generated, including both short-lived and long-lived phenomena. The short-lived phenomena include, but are not limited to:
1. Secondary electrons (low energy); PA1 2. Backscattered electrons (high energy); PA1 3. X-rays characteristic of the sample; PA1 4. "White" X-rays; PA1 5. Light (cathodoluminescence); PA1 6. Adsorbed electrons; PA1 7. Transmitted electrons; PA1 8. Auger process electrons (low energy).
Detectors are known for detecting each of the above phenomena, and there are also available mass detectors and surface potential detectors for use in an SEM.
In a conventional SEM, one of the signals identified above is detected, amplified, and displayed on a cathode ray tube (CRT) with the amplitude of the signal used to modulate the intensity of the beam of the CRT. The beam of the CRT is deflected in a raster pattern which corresponds to and is synchronized with the scanning beam of the SEM. Thus, a black and white image of the sample is presented to the operator of the microscope. The image thus created may be said to contain three bits of information at each point on the CRT--two position vectors which identify the location of the primary beam on the sample, and one brightness or intensity level which contains information about the sample. The intensity level is usually derived from the secondary electron emission, which contains topographical (slope) information. The information thus presented is in a form which is readily accepted by the human operator who, by means of his physiological and psychological systems, can rapidly assimilate the information. Only one of the many signals which are induced by the primary beam can be displayed in a conventional SEM system at any one time.
Each of the various responses of the sample to the primary beam includes unique information about the sample. For example, the secondary electron intensity contains information about the slope of the sample surface with respect to the primary beam, and this information can be used to generate an image of the sample surface. As another example, the back-scattered electron signal contains information of the atomic number of the sample, and thus can be used to provide a profile having an intensity which is representative of the chemical makeup rather than the shape of the sample being studied. In a conventional SEM system, with display, the operator can observe one of these images at a time.
Color synthesizers have been utilized by others to enhance the display image of SEM's in the past. For the most part these color imaging schemes have not increased the information content of the image but have been utilized merely to make a more esthetically pleasing picture. Color pictures have been produced photographically by means of multiple exposures of film through appropriately colored filters. Further, a system has been suggested in which three separate X-rays, each representative of a different element, are used to modulate respectively the three electron guns of a color kinescope. That is, each X-ray detector is associated with a different color, and these elements are then displayed concurrently and in color. The resultant image defines the distribution of the elements in the sample since each one is represented by a different color.
The present invention generates color images from the information available in an SEM and displays the images on a color CRT. The images are continuous in hue and cover the entire color range; and they are, of course, generated in real time, as distinguished from photographic images. According to the present invention, a first type of detector generates a signal or signals which are color encoded and fed to the tri-color guns of a CRT. The CRT may be either of the tri-dot (shadow mask) type or it may be of the tri-striped type, such as the Sony Trinitron system. In either case, the color video information is reinforced with a second video signal which provides additional information about the sample, and is coordinated with the scanning of the samples by the primary electon beam.
As a specific example, in the application of the present invention to imaging backscattered electrons in a Scanning Electron Microscope, the backscattered electrons are detected and a signal represented thereof is fed to color encoding circuits. The color encoding circuits generate three distinct signals which are coupled respectively to the blue, red and green guns of a tri-color kinescope. The encoding of the information in the signal representative of backscattered electrons may be arbitrary, but the intensity of each individual color signal should be a variable function of the input signal amplitude for at least a portion of the signal range so as to obtain a continuous variation in hue of the image thus generated.
As an example, the blue video signal of the disclosed embodiment is set at a maximum when the input backscattered signal is at a zero level, and the blue video signal decreases linearly until the mid-range of the input signal is reached, after which time the blue video signal is at shut-off. The green signal increases continuously until the mid-range point of the input signal, and thereafter decreases so as to return to a zero level green signal at the maximum input signal. The red video signal is zero until the input signal reaches its midpoint, and thereafter it increases continuously until the input signal reaches a maximum. Although this type of color encoding system may be modified, the particular one described has been shown to be useful because it presents a continuous range of hues throughout the entire range of input signal level and emphasizes the red signal (usually perceived to be associated with intensity) at the higher input levels. This type of signal has been found to be readily suited for immediate perception by a human.
The second signal may be derived from secondary electron emission from the sample, and this signal is used to vary the brightness of the color signals. The color signals modulate respectively the three beams which are deflected in a raster or pattern which corresponds to the scanning pattern of the primary electron beam in the SEM, so that the secondary electron signal (which inherently bears topographical information concerning the sample) is synchronized with the scanning primary beam. There thus appears a clear image which bears topographical information concerning the sample and superimposed therewith, a color encoded representation of other information. In the case of backscattered electron emission, such other information is representative of the average atomic number of the material or sample.
The present invention thus correlates one type of information (namely, average atomic number of a sample) with a second type of information such as topographical or spatial information in such a manner that the resultant image is ideally suited to human perception. In the example given, a continuous color hue representative of atomic number when combined with the topographical information generates an image which has the spatial or geometrical aspects of the sample correlated with a color profile representative of the average atomic number.
In other words, the present invention uses hue (i.e., a fine discrimination between the primary colors) rather than color intensity to generate the characteristic profile of the information sought. Psychologically, the ability of a person to distinguish color intensity is relatively poor, but one's ability to distinguish color variations is quite good. By varying the intensity of at least one of the primary color signals over a continuous range of input signal while using at least two primary colors at all times, the amplitude of the input signal is changed to hue variation.
Other features and advantages, as well as other uses of the present invention, will be apparent to persons skilled in the art from the following detailed description of one embodiment accompanied by the attached drawing, wherein identical reference numerals refer to like parts in the various views.